Cold front

Cold front, leading edge of an advancing mass of relatively cold air. In middle and higher latitudes of both hemispheres cold fronts tend to move toward the Equator and eastward, with the most advanced position right at the ground. At a height of about 1.5 km (1 mile), the front usually lies 80 to 160 km (50 to 100 miles) behind its surface position; thus, its slope is 1/50 to 1/100. The frontal zone, within which the rapid transition from warm to cold air takes place, is narrowest and best defined near Earth’s surface. The cold front is directly related to the polar-front jet stream and usually occurs in close proximity to its core.

A cold front is often associated with showers and thunderstorms. As it advances, often quite rapidly (50 to 65 km [30 to 40 miles] per hour), the cold air, which is relatively dense, undercuts the displaced warm air, forcing it to rise. In extreme cases, the resulting instability may lead to the formation of a squall line of severe thunderstorms and possibly tornadoes parallel to and about 80 km ahead of the surface position of the cold front. The precipitation usually stops abruptly after the front passes.

A cold front is represented on a weather map by a blue line with blue triangles whose points indicate the direction in which the cold air mass is advancing.